Ion‐exchange resin and denitrification pretreatment for determining δ¹⁵N‐NH₄⁺, δ¹⁵N‐NO₃⁻, and δ¹⁸O‐NO₃⁻ values

Abstract

Rationale There has never been a highly sensitive method for simultaneously measuring the δ15N and δ18O values of nitrate ions (NO3−) and the δ15N values of ammonium ions (NH4+) in particulate matter using denitrifying bacteria. In this study, we explored a method that combines use of an anion‐exchange resin and denitrifying bacteria to make such measurements. Methods The δ15N‐NH4+ values of samples obtained using the hypobromite and denitrifying bacteria method were measured by isotope ratio mass spectrometry. Tests (effect of flow rate, breakthrough, and acid concentration) were conducted to verify the removal of NO3− using an AG1‐X8 anion‐exchange resin for NH4+ measurements and the enrichment of NO3−. For aerosol samples, the optimized method was used to measure the δ15N‐NO3−, δ18O‐NO3−, and δ15N‐NH4+ values of atmospheric particulate matter (PM2.5, aerodynamic diameter < 2.5 μm). Results The δ15N‐NO3− and δ18O‐NO3− values measured following extraction with 1–6 mol/L HCl, at sample flow rates of 1–2 mL/min, with total anion amounts of less than 2.2 mmol, and in concentration tests were found to be in very close agreement with reagent values. The precisions and the accuracies of the δ15N‐NH4+ and δ15N‐NO3− values were in all cases less than 1‰. In addition, the accuracies for the δ18O‐NO3− values were less than 1.4‰ and generally acceptable. The δ15N‐NH4+, δ15N‐NO3−, and δ18O‐NO3− values in six PM2.5 samples were similar to those reported in previous studies. Conclusions Our proposed method for removing anions using AG1‐X8 resin, for isotopic analysis using denitrifying bacteria, and for concentrating samples containing low concentrations of NO3− will make it possible to perform high‐precision and accurate analyses easily and inexpensively. These methods are applicable not only to aerosols, but also to samples from diverse locations such as rivers, oceans, and Antarctica.

Nitrogen Cycling Across a Salinity Gradient From the Pearl River Estuary to Offshore: Insight From Nitrate Dual Isotopes

“…Nutrient concentrations of nitrite (NO 2 − ), nitrate (NO 3 − ) and phosphate (PO 4 3− ) in seawater were determined by a San ++ continuous flow analyzer (Skalar, The Netherlands), the detection limits of which were 0.1 μmol L −1 for both NO 2 − and NO 3 − , and 0.03 μmol L −1 for PO 4 3− . The chemical method for the measurement of δ 15 N‐NO 3 − and δ 18 O‐NO 3 − was used in this study, which was widely used to measure nitrogen and oxygen isotopes of nitrate in coastal water (Casciotti & Mcilvin, 2007; Chen, Bardhan, et al., 2021; Granger & Sigman, 2009; Kawashima et al., 2020; Mcilvin & Altabet, 2005; Wong et al., 2020; Ye et al., 2016), and also used in open sea with oligotrophic water (Casciotti & Mcilvin, 2007; Chen, Bardhan, et al., 2021; Ye et al., 2016). After nitrite was removed by adding sulfamic acid (Granger et al., 2008), isotopic analysis of dissolved NO 3 − was conducted following a chemical conversion method modified from Mcilvin and Altabet (2005).…”

An extraction method for nitrogen isotope measurement of ammonium in a low-concentration environment

“…This conversion is based on NH + 4 oxidation to NO − 2 by a hypobromite solution, with NO − 2 then reduced to N 2 O by a sodium azide / acetic acid solution. This second method has been applied to aerosol samples (Kawashima et al, 2021) but has not yet been applied to ice core analyses, likely due to challenges related to the smaller NH + 4 concentrations in ice.…”